Given a month in numeric form (e.g., 2 for February), how do you find the first month of its respective quarter (e.g., 1 for January)?
I read through the datetime module documentation and the Pandas documentation of their datetime functions, which ought to be relevant, but I could not find a function that solves this problem.
Essentially, what I am trying to understand is how I could produce a function like the one below that, given month x, outputs the number corresponding to the first month of x's quarter.
>> first_month_quarter(5)
4
It's a simple mapping function that needs to convert:
1 2 3 4 5 6 7 8 9 10 11 12
|
V
1 1 1 4 4 4 7 7 7 10 10 10
This can be done in a number of ways with integral calculations, two of which are:
def firstMonthInQuarter(month):
return (month - 1) // 3 * 3 + 1
and:
def firstMonthInQuarter(month):
return month - (month - 1) % 3
The first involves integer division of the month converted to a zero-based month to get the zero-based quarter, multiplication to turn that back into a zero-based month (but the month at the start of the quarter), then adding one again to make the range 1..12.
month -1 //3 *3 +1
----- -- --- -- --
1 0 0 0 1
2 1 0 0 1
3 2 0 0 1
4 3 1 3 4
5 4 1 3 4
6 5 1 3 4
7 6 2 6 7
8 7 2 6 7
9 8 2 6 7
10 9 3 9 10
11 10 3 9 10
12 11 3 9 10
The second just subtracts the position within a quarter (0, 1, 2) from the month itself to get the starting month.
month(a) -1 %3(b) a-b
-------- -- ----- ---
1 0 0 1
2 1 1 1
3 2 2 1
4 3 0 4
5 4 1 4
6 5 2 4
7 6 0 7
8 7 1 7
9 8 2 7
10 9 0 10
11 10 1 10
12 11 2 10